Field of the Invention
The present invention relates to a fixing device included in the image forming apparatus using electrophotographic technology.
Description of the Related Art
In general, fixing devices mounted in an image forming apparatus, such as a copying machine and a laser printer, convey a recording medium through a nip portion formed by a first fixing member and a second fixing member that are in pressure contact with each other and heat-fix an unfixed toner image onto the recording medium.
Among such fixing devices, some fixing devices include a pair of pressure mechanisms that urge both ends of the first fixing member against the second fixing member using the elastic force of a helical compression spring so that the first fixing member and the second fixing member to are in pressure contact with each other. To improve the pressure balance between the two pressure mechanism, a configuration that aligns the winding end positions of the helical compression springs disposed at both ends has been developed (refer to Japanese Patent No 3501616). However, the fixing device described in Japanese Patent No. 3501616 has the following issues. That is, by aligning the positions of the winding ends of the helical compression springs, the pressures at both the ends of the fixing member are forced to be the same. In such a technology, since at the ends of the helical compression spring, the protrusion level of the spring winding end of the coil in the axial direction of the coil is the highest, the portions in the vicinity of the spring winding ends receive a large reaction force from spring supporting portions, as indicated by outlined arrows illustrated in FIG. 11. Each of arrows in FIG. 11 indicates the magnitude of a reaction force received by a helical compression spring 87 from a spring support member (the length of the arrow) and the direction of the reaction force (the direction of the arrow). According to the fixing device described in Japanese Patent No. 3501616, the helical compression spring 87 receives reaction forces F11 and F12 in one of two spring support areas thereof in the cross section that passes through an axial line 87s of the helical compression spring 87 and reaction forces F13 and F14 in the other spring support area. The reaction force F11 in the vicinity of the spring winding end is larger than the reaction force F12. The reaction force F14 in the vicinity of the spring winding end is larger than the reaction force F13. Accordingly, the helical compression spring 87 does not receive a uniform reaction force from the supporting portion. Consequently, a force that rotates the helical compression spring 87 is easily generated. As a result, the direction of action of a force Fs of the helical compression spring 87 is inclined from the direction of a pressure Ft applied in the nip portion and, thus, loss of the pressure applied in the nip portion easily occurs.
In addition, a configuration that corrects the balance between the reaction forces exerted on a helical compression spring by cutting and grinding the spring terminals has been developed. However, if the helical compression spring having cut and ground spring ends is employed in fixing devices, the cost increases. In addition, the following issue arises. That is, the helical compression spring having cut and ground spring ends has a small thickness of the coil in the vicinity of the winding end and, thus, the rigidity easily decreases. If a high load is imposed on the thin coil portion, the spring deforms. As a result, the pressure in the nip portion decreases.